1. Field of the Invention
The present invention relates to a method of constructing a shaft with improved operating efficiency.
2. Description of the Prior Art
When a scheduled building construction site has poor ground, the ground is generally given improvement treatment prior to the construction. Such ground treatment usually consists of hardening the soil by permeating it with a hardening agent. One method that has come into extensive use in recent years is the injection mixing method in which air and hardener are forced into the ground under high pressure to form a pile-shaped solid mass. The apparatus for implementing the injection mixing method consists of a high-pressure pump for pumping the cement or the like that constitutes the hardener; a compressor to supply the air; and a machine to operate a double-walled boring pipe the tip of which is provided with a nozzle.
The high-pressure pump is usually a plunger-type pump, which is used to pump the hardener from a mixer under very high pressure. The machine mixes the air and hardener and injects it into the ground from the nozzle of the double-walled boring pipe. For this, the boring pipe is inserted into a borehole in the ground, and for the injection process is revolved while being raised up from the bottom part of the hole. A columnar mixed region of soil and hardener is thereby formed in the ground and with the setting of the hardener forms into a pile-shaped solid mass.
The injection mixing method is widely employed because it provides good operational efficiency. However, the diameter of the columnar mixed region is dependent on soil condition and pump performance, and in sandy soil or clay the standard effective diameter is limited to 2000.+-.200 mm.
FIG. 6 illustrates the method generally employed to construct a shaft having an inside diameter of 3500 mm, as shown in FIG. 5, using the injection mixing method. The double-walled boring pipe of the operating machine is inserted into the bottom 101 of the shaft 100 in sequence from al to a4 to inject the hardener and form a columnar mixed region 102. The double-walled boring pipe is then inserted from b1 to b10 around the perimeter of the shaft 100 to form a columnar mixed region 103. After the hardener has set, the inner portion of the mixed region 102 is excavated to produce the shaft 100 having the prescribed diameter.
With the conventional shaft construction method described above, the operation of removing the portion indicated in the drawing by shading to produce the requisite diameter D is rendered difficult owing to the fact that the hardener has set. This lowers operating efficiency. In addition, the need to form numerous columnar mixed regions 102 and 103 requires much time.